The present invention broadly relates to the treatment of aqueous materials and, more specifically, pertains to a new and improved method for drying sludge.
Generally speaking, in the practice of the invention for drying a sludge containing organic substances and obtained subsequent to the dewatering of a suspension, there is utilized a fluidized bed dryer having a heated sand layer which is fluidized by means of a gas stream or current for gaining as a product the dry matter of the sludge.
For such drying operations there can be provided all suspension, sludges, pastes and filter cakes from an almost endless variety of dewatering machines and installations. Such materials to be dried also include sewage sludges from municipal or industrial waste water or sewage, such sewage sludges originating from sewage treatment or clarification plants charged with such waste water or sewage.
These sludges are mechanically dewatered. Further processing, utilization or disposal thereof presupposes a thermal drying operation. The pasty sludges contain 40% to 60% water depending on the degree of dewatering, and such water content substantially impairs the handling or transport of sludge and the methods for utilization or disposal of the sludges. Thermal drying improves or augments the possibilities of utilizing or applying these sludges and/or reduces the resulting amount of sludge to be disposed of.
According to a method known to the art there is utilized a direct rotary or drum dryer. The sludge is fed, sometimes subsequent to pretreatment depending on the consistency, into a rotating drum inclined towards the outlet thereof and the sludge travels to the discharge or outlet end by continuous rolling motion within the rotating drum. At the same time, hot air or hot flue or exhaust gas streams through the rotating drum in co-current flow or contra-current flow and thereby absorbs the moisture of the sludge. This known installation or plant requires a relatively high constructional expenditure for its mechanical or machine components and employs a relatively energy-consuming drying operation, the humid and contaminated exhaust air from the rotary or drum dryer requiring complicated and uneconomical cleaning or purification for appropriate limitation of its emission.
Other drying apparatus known to the art are direct sand fluidized bed dryers. Hot air or hot flue or exhaust gases stream through a sand layer from the bottom toward the top, thus causing a fluidization of the sand filling or charge. An inflow floor or bottom provides for a uniform distribution of the inflowing hot gases. Several pumps force the sludge by means of jets or nozzles directly into the fluidized sand layer. The jets or nozzles are arranged just above the inflow floor or bottom, i.e. arranged within the lower quarter of the sand layer. The sludge dissipates or transfers its moisture to the through flowing hot gas which has to be cleaned or purified after discharge from the fluidized bed dryer.
Furthermore, there are known indirect contact dryers in which the heating of the sludge is indirectly effected by means of heating surfaces. Depending on the type of dryer, these heating surfaces possess the form of discs, paddles, rolls and the like. The steam-heated or oil-heated heating surfaces heat the sludge until the moisture thereof finally evaporates, a ventilator or blower sucking away the resulting exhaust vapors and compressing the latter to condensate. The sludges are either applied to the heating surfaces in a thin layer and then abrased or scraped off, or the heating surfaces are moved or stirred in the product to be dried.
A further drying apparatus known to the art from U.S. Pat. No. 4,330,411, granted May 18, 1982, is the indirect fluidized bed dryer which is also utilized to perform the method for drying sludge of the present invention. According to this known sludge-drying method utilizing an indirect fluidized bed dryer, a sludge granulate in a fluidized bed undergoes throughflow of superheated exhaust vapors in circulation from the bottom toward the top and is thereby fluidized. By virtue of an inflow floor or bottom provided with jets or nozzles, the gas is uniformly distributed across the entire surface of the fluidized bed dryer, thus ensuring a uniform fluidization of the sludge granulate. In this fluidized layer there are provided heating surfaces, for instance heat exchangers, which indirectly transfer the required drying energy to the sludge granulate. Such heating surfaces possess different forms, such as bare or flat tubes, finned or externally ribbed tubes, plates and the like, which are heated by means of steam or any other suitable heat-carrier. A granulator or granulating machine produces a moist stable granulate by mixing dewatered sludge and a portion of already dried sludge granulate. This moist stable granulate is fed into the fluidized layer where the granulate moisture is absorbed by the throughflowing superheated exhaust vapors. The dried granulate leaves the fluidized bed dryer via an overflow or separating weir or through a discharging apparatus. A portion of this dried granulate returns as add-back material to the granulator or granulating machine where dewatered sludge is added to prepare or yield the moist stable granulate. The exhaust vapors leaving the fluidized bed dryer also entrain fine-grained product particles and dust which are precipitated or separated in a cyclone or filter and discharged to the granulator or granulating machine. The amount or quantity of water evaporated during the drying process is withdrawn from the recycling system in the form of exhaust vapors and condensed or thermally heated.
With the heretofore employed drying methods utilizing direct rotary or drum dryers, direct sand fluidized bed dryers or indirect contact dryers it has been possible to achieve the expected advantages and results of the drying operation only by means of a relatively complicated mechanical-thermal process, such that the economic viability of the overall operation of drying sludge has been only partially taken into consideration.